Ferroelectric liquid crystals (FLCs) are liquid crystals that have spontaneous polarization and are ferroelectric. It is known that when liquid crystals having a permanent dipole moment in a direction perpendicular to the molecular long axis direction form a smectic phase, a layer structure is formed. When the molecular long axes in the layer are tilted, and the liquid crystals form a chiral smectic C (hereinafter abbreviated to SmC*) phase, the permanent dipole moments cannot be cancelled each other out. Thus, the liquid crystals have spontaneous polarization and exhibit ferroelectricity. Application of a voltage to the liquid crystals aligns the permanent dipole moments along the electric field direction and also aligns all the liquid crystal molecules. Ferroelectric liquid crystals having the SmC* phase are widely used in display elements. Ferroelectric liquid crystals are smectic liquid crystals, such as p-decyloxybenzylidene p′-amino 2-methylbutyl cinnamate (DOBAMBC), that have optical activity (chirality). DOBAMBC was designed and synthesized by R. B. Meyer et al. in 1975. Even the addition of an optically active compound having no liquid crystallinity (which is not a liquid crystal compound) can form the SmC* phase. In this case, mother liquid crystals having an achiral smectic C (hereinafter abbreviated to SmC) phase are generally used.
Among smectic phases having a layer structure, the alignment direction of liquid crystal molecules in the SmC* phase is tilted at an angle to the layer normal. The tilt angle (the azimuth angle of direction) to the layer plane can vary slightly layer by layer, thereby forming a helical molecular alignment.
Ferroelectric liquid crystals characteristically have 10 times or more higher responsivity than display elements based on nematic liquid crystals. Ferroelectric liquid crystals first applied to display elements are surface-stabilized ferroelectric liquid crystals (SSFLC), which were devised by Clark and Lagerwall. After that, ferroelectric liquid crystals have been actively studied.
In SSFLC, liquid crystals are aligned using a substrate subjected to parallel alignment treatment such that the layer normal is parallel to the substrate face of the cell (homogeneous alignment), and the thickness of the liquid crystal layer is reduced to loosen a helix, which makes it difficult to tilt liquid crystal molecules on the substrate face. This limits the azimuth angle of direction to two ranges and creates alignment memory (bistability) by the action of surface stabilization, thus resulting in a black-and-white binary display element having the memory. Although such a display element has high-speed responsivity, it is difficult to display gray-scale images using such a binary display element. Furthermore, when heated liquid crystals disposed between substrates are cooled and form a SmC* phase, the liquid crystals are tilted, and the layer interval is reduced. This results in a chevron structure in which the layer plane bends to form a chevron shape, tends to cause a zigzag defect, and results in difficulty in achieving high-contrast. Thus, alignment of liquid crystals has been actively studied to apply the liquid crystals to display elements (see Non-Patent Literature 1).
Distorted helix (or deformed helix) ferroelectric liquid crystals (DHFLC) are also known to have a non-limited range of azimuth angle of direction in order to solve difficulties in gray-scale display resulting from bistability (see Non-Patent Literature 2). In such a method, the helical pitch of FLC is sufficiently reduced and is smaller than the thickness of a liquid crystal layer between substrates. Although such ferroelectric liquid crystals have uniaxial birefringence having an axis in the helix axis direction in the absence of an applied voltage, such ferroelectric liquid crystals under an applied voltage gradually deviate from the helical arrangement of the liquid-crystal alignment and have changes in birefringence, thereby achieving continuous gray-scale displays. However, in DHFLC described in Non-Patent Literature 2, because the layer is perpendicular to the substrate face, that is, the layer normal direction is generally parallel to the substrate face, there is a problem with respect to the viewing angle of the display element.
One of methods for improving the viewing angle of a ferroelectric liquid crystal display element is a technique that has been developed for nematic liquid crystal display elements and has been applied to ferroelectric liquid crystals. In nematic liquid crystals, an electric field perpendicular to the substrate is used in a homeotropic alignment mode, and homeotropic alignment of liquid crystal molecules is utilized to improve the viewing angle. In-plane switching (IPS) is a method for improving the viewing angle by switching homogeneously aligned liquid crystal molecules using a horizontal electric field parallel to the substrate. The homeotropic alignment and IPS can be used in combination. For example, Non-Patent Literatures 3 and 4 describe a liquid crystal display element in which a horizontal electric field is applied to homeotropically aligned DHFLC using in-plane electrodes composed of a pair of interdigitated electrodes disposed on a lower substrate. Non-Patent Literature 5 describes a light modulator in which readout laser beams enter homeotropically aligned DHFLC along various directions while a horizontal electric field is applied to the homeotropically aligned DHFLC. However, in order to achieve a contrast as high as the VA mode developed in nematic liquid crystals using ferroelectric liquid crystals, it is necessary to prevent the formation of a schlieren texture and focal conic alignment defects characteristic of SmC* in homeotropic alignment. To this end, ferroelectric liquid crystals may be homeotropically aligned at a short helical pitch of 400 nm or less. In this case, owing to a high melting point and a narrow temperature range of the SmC* phase as a result of a high concentration of a chiral dopant, the liquid crystal display element has a limited operating temperature range. Furthermore, because a high electric field strength is required to loosen the short-pitch helix, the liquid crystal element requires a high driving voltage. Furthermore, the number of components in the composition needs to be increased to lower the melting point. This reduces the productivity of the ferroelectric liquid crystal display element and becomes a barrier to the practical use of the ferroelectric liquid crystal display element also from an economic point of view. Furthermore, in SSFLCD, it is difficult to recover from a disturbance of alignment resulting from deformation of an element, for example, caused by an external pressure. Although an improvement of the cell structure or polymer stabilization have been proposed, it is not feasible on a large scale.